Electric valve converting system



, 0t.22,'1940. H, Q M D 2,219,200

ELECTRIC VALVE CONVERTING SYSTEM Filed June 7, 1939 4 Sheets-Sheet 1 Fig.1.

Inventnr: HeUmuihO. Schmidt,

H ttorneyw I Oct. 22, 1940. 0 sc ELECTRIC VALVE CQNVERTING' SYSTEM Filed June 7, 1939 4 Sheets-Sheet 2 Inventor Helh'nutho. Schmidt, by

His ttorney.

Oct. 22, 940. I H O scH 2,219,200

I ELECTRIC VALVE CONVERTING SYSTEM Filed June 7 1939 4 Sheets-Sheet 3 Fig.6.

AAAAAAAAAAAAAAAAAAAAAAA/\AAAAAAAAAAAAAAAAAAAA/MAI VV VVV VVV VVV VVV vvvvvvvvvvvvvvvvvvvvvvvvv Inventor: HellmuthO. Schmidt,

His/Attorney.

Oct. 22, 1940. O. SCHMlDT 2,219,200

ELECTRIC VALVE CONVERTING SYSTEM Filed June '7, 1939 4 Sheets-Sheet 4 Fig.8.

.ID \MMMMMMMAAMMMMMMMMMMMMMMAMMMMMAAM WWW 1VWV WWVVVVVVVWWWWWV\I WVVWVVVVVVV\JWWVWW Inventor: Hellmuih O. Schmidt,

10 1/ 6M Hiz zttorney.

UNITED STATES PATENT OFFICE ELECTRIC VALVE CONVERTING SYSTEM Hellmuth 0. Schmidt, Berlin, Germany, assignor to General Electric Company, a corporation of New York Application June 7, 1939, Serial No. 277,936 In Germany June 23, 1938 15 Claims. (Cl. 172--281) My invention relates to electric valve convertulation of the output voltage may be obtained in ing systems and it has particular relation to apa simple and economical manner. Accordingly, paratus for changing currents of relatively low I have provided a frequency changing apparatus frequency into currents of relatively high frefor supplying an induction heating apparatus quen'cy, especially in connection with inductive wherein complete regulation of .the output volt- Gil heating systems. age may be had without any of the disadvan- Polyphase' alternating currents of low fretages inherent in known apparatus. quency have been converted into single phase It is an object of my invention to overcome alternating currents of high frequency by operthe disadvantages above noted in connection with ating at the high frequency a plurality of bipha-se electric valve converting apparatus for use in 10 rectifiers, corresponding in number to the numconnection with frequency changing systems. ber of phases of "the low frequency alternating It is another object. of my invention to procurrent circuit, and inductively coupling an outvicle a new and improved electric valve frequency put winding to the windings of the biphase recchanger in which complete regulation of the outtifiers for supplying the higher frequency current put voltage may be obtained. 15 necessary to operate an induction furnace. It Still another object of my invention is to prois often desirable to be able accurately to control vide an electric valve converting system for supthe power supplied to induction furnaces, and plying the high frequency alternating current where they are used for alloying metals it is esnecessary to energize an induction heating apsential that regulation of this power be possible. paratus and wherein regulation of the power 20 Frequently in alloying the process is carried out supplied to such heating apparatus is obtained. according to a time temperature graph and in It is a further object of my invention to proorder that this graph may be followed the power vide an electric valve conventing system of the supplied to the furnace must be controllable. type wherein a plurality of biphase rectiflers are The electric energy consumed by an induction used to convert a low frequency polyphase alter- 25 furnace is proportional .to the square of the voltnating current into a high frequency single phase age supplied to the furnace and hence where an alternating current wherein complete control of electric valve frequency changer supplies the enthe power output is possible over a very wide ergy, control of the power supplied may be obrange of operation.

tained by regulating the output voltage of the a It is another object of my invention to control 30 frequency changer. Arrangements have been the power output of an electric valve frequency proposed for regulating the output voltage in fre- Ch g g ppara us tho in y Way disturbquen-cy changers supplying energy to an inducg t e o al Operation of e ppa tion furnace. For example, it has been suggested It is another object of my invention .to control to provide taps on the supply transformer so as the p r Output Of an electric Valve frequency 35 to control the magnitude of the voltage applied ha in appa a s by tw di r tmeanswhereto the frequency changing apparatus but such by both a fine and coarse regulation ay be obregulation, which must take place under load, minedrequires tap changing equipment which is not as t s an e Object Of my invention o p o simple to build and to operate as is desired. It v a on l m ns for n el ric valve fre- 40 has also been suggested to provide an arrangequency changing apparatus wherein the frement wherein variable portions of the supply quency changer may be effectively cut out of the voltage r d t make up the high frequency circuit while supplying a load without elaborate output voltage and when increased output voltand expensive Switch ng means.

5 age is desired, a larger portion of the supply volt- In accordance with the illustrated embodiage is used and hence the power output is inmerit of my invention I provide an electric valve creased. This arrangement has several disadfr qu y Changing apparatus wherein a t l' e vantages, among them being that some compulp ase-a te at C e S pp y voltage is sory commutation scheme is necessary, and furt fied th oug a plurality D e e fi thermore only a relatively slight frequency one for each phase of the alternating cur- 50 change is possible since the output frequency can rent supp y, and wherein a i qu y only be multiplied by the number of phases in output voltage is obtained from a winding inducthe supply circuit. It would be desirable, theretively related to the windings of the biphase recfore, to provide a frequency changing apparatus tifiers which operate at the high frequency defor supplying an induction furnace wherein regsired at the output. Complete control of the 55 output voltage of the frequency changer is possible by means of auxiliary tubes in the grid circuits of the biphase rectifiers whereby it is possible to control the period of use of each cycle of each phase of the alternating current supply voltage impressed on the biphase reoti-fiers and thus control the power output to the induction furnace. By changing the frequency of the alternating po-. tentials supplied to the biphase rectifier control electrodes a second means of controlling the output power is provided. Also by simple control means the frequency changer may be cut out of the circuit even while supplying full load.

For a better understanding of my invention reference may be had to .the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims. Fig. 1 of the accompanying drawings diagrammatically illustrates a frequency changer of the type to which my invention is applied and shown symbolically, and Fig. 2 represents certain operating conditions of this frequency changer; Fig. 3 illustrates in more detail an embodiment of my invention applied to the frequency changer illustrated in Fig. 1, and Figs. 4, 5, 6 and 7 represent certain operating conditions of the arrangement shown in Fig. 3; Fig. 8 diagrammatically shows another embodiment of my invention as applied to the frequency changing system illustrated in Fig. l, and Fig. 9 represents certain operating characteristics of the arrangement shown in Fig. 8.

Referring now to Fig. 1 of the accompanying drawings, I have shown diagrammatically the power circuits of an electric valve frequency changing apparatus with symbolically represented control circuits in accordance with my invention for transmitting energy between a polyphase alternating current circuit l and a single phase alternating current load circuit I l of higher frequency than that of the supply circuit l3. Energy is supplied from alternating current circuit It to the primary winding I2 of a threephase transformer l3 having a Y-connected secondary winding iii. A plurality of biphase rectlfiers is, It and H are provided which comprise inductive windings l8, l9 and 20, each including a pair of end terminals and midtap. The midtaps of each of the windings l8, I9 and 23 are connected, respectively, to one of the phases of the Y-connected secondary winding H of transformer l3. Electric discharge valves 2|, 22, 23, 26, 25 and 26 are associated with each of the end terminals of windings l8, l9 and 20. Each of the electric discharge valves is provided with an anode 27, a cathode 28 and a grid or control electrode 29. The electric discharge valves 29, 22, 23, 2%, 25 and 26 may be any of the types of electric valves well known to those skilled in the art, although I prefer to use electric valves of the type embodying an ionizable medium such as a gas or a vapor. The cathodes 28 of electric valves 2i to 26 are all connected together and form the positive terminal of direct current circuit 39 which includes a reactor 3! connected to the neutral of the primary winding Id of supply transformer l3 through a suitable conductor 32. Although I have illustfied electric valves 2! to 26 as of the type each comprising a. separate envelope containing an anode, a cathode and a control electrode, it will be understood that these valves may all be combined into a single valve having a single cathode and a. plurality of anodes and grids or control electrodes. Commutating capacitors 33 are connected across each of the biphase rectifier windings l8, l3 and 20 in order to supply the commutating voltage necessary to transfer the current from one valve to the other of each of the biphase rectifiers. The control electrodes 29 of electric discharge valves 2! to 26 are supplied with an alternating potential from a high frequency alternating current source indicated by the terminals 34 of a frequency corresponding to that desired at the load circuit I i. This high frequency alternating potential from the terminals 34 is applied to each of the control electrodes 29 through grid transformers 35 having a midtap 36 provided on the secondary winding thereof. The midtaps 33 are connected together through suitable current limiting resistors 37 and a capacitor 38 to the oathodes 23 of the discharge valves 2| to 26. The capacitor 38 is connected to provide a self biasing arrangement for maintaining a negative bias on the control electrodes 29 of each of the electric valves 2| to 23 which are idle during a particular interval of time. Capacitor 38' is charged from the small positive ion current which flowsthrough the grid circuit of the particular electric valve conducting current at any particular instant.

The load circuit for the electric valve frequency changer comprises secondary winding 33 mounted on a common core in with each of the windings l8, l9 and 2B of biphase rectiflers I5, l6 and ll. Due to the grid control each biphase rectifier operates at the high frequency desired for the load circuit H, i. e., current commutates at the output frequency from one tube to the other in the particular biphase rectifier carrying current. A corresponding high frequency voltage will therefore be induced in winding 39. In addition the low frequency alternating current supply voltage will also be induced in winding 33 and there results a voltage which comprises the superimposition of the low and high frequency voltages induced in secondary winding 39. This voltage is supplied to the heating coil ll of the induction furnace 52 schematically represented in Fig. 1 by a crucible. In order to produce a unity power factor load for the electric valve 'frequency changing apparatus a variable capacitor 43 is provided across secondary winding 39 which is adjusted from time to time during the operation of the furnace so as to compensate for the variable wattless current demand of the furnace coil 4!. Elements 3% to 53 symbolically represent in Fig. 3, and described hereinafter.

The operation of the frequency changing apparatus illustrated in Fig. 1 disregarding for the moment the control elements symbolically shown at 6% to it may best be understood by reference to Fig. 2 where A, B and 0 represent the useful portions of the three-phase alternating current voltages applied respectively to the biphase rectifiers l5, l6 and W. D indicates the high frequency alternating current voltage supplied to the furnace coil ill which has an amplitude variation corresponding to that of the three-phase supply voltage. When the alternating current potential of the supply circuit l0 isimpressed on the biphase rectifiers l5, l or ll the particular one of these rectifiers upon which the highest positive potential is impressed, will begin operation. Assume for example that biphase rectifier l5 begins to operate; then for substantially 120 electrical degrees with reference to the supply voltage, biphase rectifier l5 will transmit the energy from the alternating our-- .my control circuit illustrated in greater detail rent supply circuit N to the direct current circuit 30 and current will alternately flow first through electric valve 2| and then through electric valve 22, the changes occurring at the frequency of grid potential 34. This alternate conduction of electric valves 2| and 22 at the high frequency results in a current flow in the two halves of winding l8 which causes a high frequency flux change in core 46 and, of course, induces a corresponding high frequency voltage in secondary winding 39. After biphase rectifier l5 has been conducting current for substantially 120 electrical degrees with reference to the supply voltage, the positive potential impressed upon biphase rectifier i6 becomes higher than that impressed on biphase rectifier 5 and hence biphase rectifier |6 begins to conduct the current alternately through discharge valves 23 and 24 at a frequency determined by the frequency of grid potential 34 and current ceases to flow through biphase rectifier i5 after commutation is complete. The useful period of each biphase rectifier during one cycle of supply voltage is represented by a in Fig. 2 while b represents the average amplitude approached by the high frequency output voltage D when inductance 3| in the direct current circuit is large.

Fig. 3 of the accompanying drawings diagrammatically illustrates in greater detail an embodiment of my invention applied to the frequency changer illustrated in Fig. l and symbolically shown by elements 44 to 49. The arrangement of the main power circuits illustrated in Fig 3 is substantially the same as shown in Fig. 1 and the same reference numerals are retained for the corresponding parts. As illustrated in Fig. 3, I provide a control circuit for each of the electric discharge valves 2| to 26 of the biphase rectiflers i5, i6 and II which is superimposed upon 0 the main control circuit comprising alternating voltage 34, grid transformer 35 and the biasing capacitor 38. In the grid to cathode circuit of the biphase rectifiers i5, I6 and I1 I provide reversely connected auxiliary discharge valves 50, 5| and 52, respectively, each connected in series with grid resistance 31 and comprising an anode 53, a cathode 54 and a control electrode 55. Pushbutton switches 56, 51 and 58 are provided in series with each of the auxiliary discharge valves 50, 5| and 52, respectively, the purpose of which will be hereinafter described. In parallel with electric discharge valve and pushbutton switch 56 I provide a circuit including battery 59,

resistance 60 and secondary winding 6| of a potential transformer 62, the primary winding 63 of which is connected across a resistor 64 in the alternating voltage supply line connected to the D next succeeding biphase rectifier to become conductive, namely rectifier l6. Battery 59 aids capacitor 3'8 in impressing a negative bias on the control electrodes 29 of the electric discharge valves 2| and 22 when they are not conducting current. A battery 65 and resistance 66 are included in the grid circuit of control electrode 55 of auxiliary discharge valve 50, the battery 65 normally impressing a negative bias voltage upon the grid 55. A phase shifter 61 energized from alternating current supply circuit i0 impresses a voltage across resistance 66 so as to overcome the negative bias on grid 55 and allow auxiliary discharge valve 50 to become conducting during the time when the supply voltage impressed on winding l6 of biphase rectifier 5 is positive. By means of phase shifter 61 it is possible to retard or advance the excitation of grid 55 of auxiliary discharge valve 50 and thereby retard or advance the interval at which electric discharge valve 50 becomes conductive and hence also the interval when valves 2| and 22 may become conducting, for until auxiliary discharge valve 50 has become conductive a negative bias equal tov the voltage of battery 59 plus the voltage on condenser 38 maintains the grids 29 of electric discharge valves 2| and 22 negative and thereby prevents these valves from becoming conductive. When the current commutates from biphase rectifier |5 to biphase rectifier |6 the initiation of a flow of current through resistor 64 will cause an impulse voltage to be induced in the secondary winding 6| of potential transformer 62 which will impress a high negative voltage impulse on the anode 55 of electric discharge valve 50 relative to the cathode 54, overcoming the voltage of battery 59 and thereby forcibly extinguishing the current being conducted by valve 50. A peaked wave shape may be given to the potential obtained from transformer 62 by properly proportioning the time constant of the circuit. I have provided an electric discharge valve 68 having an anode 69 and a cathode 10 connected across electric discharge valve 50, and arranged with anode and cathode positioned in the same direction as electric valves 2| and 22 but reversed with respect to auxiliary valve 50 so as to absorb, by becoming conductive, the high voltage impulses induced by the secondary 6| of potential transformer 62.

A control circuit similar to that just described for biphase rectifier I5 is provided for rectifiers i6 and H. A potential transformer '52 has its primary winding 13 connected across resistance 14 in the alternating current supply line to rectifier while the secondary winding H is connected in the grid control circuit of rectifier |6 across auxiliary electric discharge valve 5| and pushbutton 51. Similarly a potential transformer 15 has is primary winding 16 connected across resistance 'il in the alternating current supply line to rectifier i5 while the secondary winding 18 is connected in the grid control circuit of rectifier across auxiliary electric discharge valve 52 and pushbutton 53. The remaining parts of the control circuits of biphase rectifiers i6 and I! are characterized by the same reference numerals as the corresponding parts of rectifier :35. By means of the grid control just described for the biphase rectifiers the period of use of these various rectifiers relative to the positive half cycle of the alternating current supply voltage may be increased or decreased so as to control the magnitude of the output voltage and hence the power supplied to the induction furnace load.

In order to understand the operation of the embodiment of my invention illustrated in Fig.

3, reference may be had to Fig. 4 which is simil'ar to Fig. 2 except that the phase positions of the phase shifters 61 have been changed so as to retard the instant during each cycle that the electric valves 2| to 26 become conducting. This angle of retardation with respect to the fully modulated operation of the biphase rectifiers illustrated in Fig. 2 is shown as ,8 in Fig. 4. As in Fig. 2 the period of use of each cycle of each phase of the supply voltage is illustrated by a and the average value b of the output voltage D has been reduced by an amount Ab thus reducing the power supplied to the furnace. When the alternating current supply causes the maximum positive potential to be impressed acros biphase rectifier |5 this rectifier is conducting current and rectifiers I6 and H are idle. High frequem cy alternating potential 34is impressed upon the grids 29 of electric discharge valves 2| and 22. However, unless auxiliary electric discharge valve 50 is conductive and push button switch 56 is closed, high frequency grid potential 34 is ineffective since the grid to cathode circuit is not complete. Since the grid 55 of auxiliary discharge valve 50 is supplied with an alternating potential of the same frequency as the supply potential In, auxiliary discharge valve 50 may be made conductive during substantially 120 electrical degrees with reference to the supply voltage or during the maximum time that biphase rectifier l may conduct current. However, by means of phase shifter 6?, as is well understood by those skilled in the art, the control electrode 55 of electric valve 50 may be maintained negative at the voltage of battery 65 for some time after biphase rectifier I5 would ordinarily become conducting if the arrangement disclosed in Fig. 1 exclusive of control members 64 to 49 were present. Hence, by controlling the position of phase shifter 61 the time at which electric discharge valve 50 becomes conducting may be controlled and since electric discharge valves 2| and 22 are maintained non-conductive as long as the negative potential of battery 59 and condenser 38 are impressed upon electrodes 29, biphase rectifier cannot conduct current until battery 59 is shunted by the current discharge in valve 50. When the current commutates from biphase rectifier l5 to biphase rectifier l6 the initiation of the current flow through resistor 64 causes a voltage impulse to be induced in the secondary winding 6| of potential transformer 62 and this voltage impulse is of such magnitude as to overcome the battery voltage 59 and extinguish the current discharge through electric valve 50 by impressing a high negative potential on the anode 53 relative to the cathode 5a of electric discharge valve 5!). Auxiliary discharge valve 68 by becoming conductive prevents the high voltage impulse produced by potential transformer 62 from reaching dangerously high values. Since the discharge across auxiliary valve 50 has been extinguished the negative bias of battery 59 and conductor 38 is again impressed on the grids 29 of the electric discharge valves 2| and 22, thus enabling the grids 29 of these valves to again obtain control whereby the current is commutated to biphase rectifier Hie The same process is repeated for the operation of biphase rectifier I6 when the current is commutated to biphase rectifier It is, therefore, possible by adjusting phase shifters 61 of biphase rectifiers I5, l6 and I! to obtain an output voltage similar to that illustrated in Fig. 2 when maximum power output is obtained. f the phase shifters B1 are positioned so that electric discharge valve 50 is prevented from becoming conductive for an angle 5 as illustrated in Fig. 4, the reduced output voltage illustrated in Fig. 4 will be obtained. It will be understood by those skilled in the art that any desired output voltage may therefore be obtained by this arrangement and no interference with the high frequency control means of the biphase rectifiers will occur.

An additional regulation of the output voltage of the frequency changer may be obtained by varying'the frequency of grid potential source 34. A change in frequency of the alternating potential impressed across the parallel connected load coil 4| and capacitor 43 will cause a change in the power taken by the load by virtue of resultant impedance change. By this arrangement the control frequency may be changed in fairly large steps giving power output control in fairly large steps. A fine control may therefore be obtained for each step by properly positioning the phase shifters 61. The combination of Varying the frequency of the grid potential and also varying the useful portion of the supply voltage results in a control of the power output to the induction furnace over a very wide range.

Pushbutton switches 56, 51, and 58 will serve to break the current flow through auxiliary electric discharge valves 50, 5| and 52, respectively, and since this current is normally very small, the electric valve frequency changer may be made inoperative at any time by merely opening pushbutton switches 56, 51 and 58, at which time the negative bias of battery 59 is impressed on all the control electrodes 29 of electric discharge valves 2| to 26. When this electric valve frequency changer is used to supply an electric furnace this simple means of disconnecting the frequency changer is very desirable if it is necessary to vary capacitance 43 to compensate for the variable wattless current demand of the furnace coil 4|. No expensive switches for breaking the main power circuit are necessary and yet by opening pushbutton switches 56 to 58 the furnace coil is entirely deenergized.

Fig. 5 shows the anode voltage E as it would appear across electric' discharge valve 2| of biphase rectifier |5 when the frequency changer is supplying the maximum power corresponding to the condition illustrated by Fig. 2. The three phase alternating current supply voltages A, B and C are also shown. The positive half cycle of supply voltage A is the useful period as far as the operation of the biphase rectifier I5 is concerned and this period is indicated by a in Fig. 5. Observing the anode voltage E, namely the voltage across discharge valve 2|, it is seen that the conducting period is indicated by the heavy horizontal line of almost zero voltage which is immediately followed by the negative deionization peak or commutating voltage obtained from condenser 33. Then follows the rise of the anode voltage during the succeeding half cycle with reference to the high frequency output circuit during which time electric discharge valve 22 is conducting current and then the next succeeding discharge period 180 degrees later when discharge valve 2| again begins to conduct current. As soon as the alternating current supply potential B of the next succeeding phase becomes greater than the positive potential A, biphase rectifier l6 begins to conduct current. Because of the inductive rela tion of windings I8, I9 and 20 the anode voltage E across electric discharge valve 2| is a high frequency alternating potential superimposed upon the alternating current low frequency supply potential impressed on biphase rectifier l5 as shown during the period c in Fig. 5. Also during the range c when biphase rectifier I5 is idle it is observed that the anode voltage of valve 2| becomes positive at times. If the grid 29 also became positive then valve 2| would conduct during the time when it should be idle. However, condenser 38 functions so as to prevent this from occurring.

Fig. 7 shows the anode voltage E as it would appear across electric discharge valve 2| of hiphase rectifier |5 when the phase shifters 61 have been adjusted so that a retardation of the grid control of electric discharge valve 50 amounting to an angle ,9 has been obtained similar to the operating conditions shown in Fig. 4. Except for the positioning of the phase shifters the same operating conditions illlustrated by Fig. are also illustrated in Fig. 7. The anode volt age E during the useful period when biphase rectifier i5 is conducting is considerably decreased in Fig. 7 as compared with Fig. 5.

Fig. 6 is an oscillogram representing the various current and voltage conditions in the embodiment of my invention illustrated in Fig. 3 when the operating conditions represented by Figs. 4 and 7 occur. 11 illustrates the impulse current flowing in the secondary winding 18 of potential transformer for extinguishing the discharge current through auxiliary discharge valve 52 of biphase rectifier I! to commutate the current from rectifier "II to rectifier I 5. I2 represents the positive ion current flowing through the grid circuit of electric discharge valves 2i and 22. F represents the potential across electric valve 50, while G represents the grid potential of auxiliary discharge valve and contains the negative biaspotential of battery and the voltage delivered by the phase shifter 61 across resistor 66. E indicates the po tential across discharge valve 2| and is identical with the voltage E shown in Fig. 7. D represents the high frequency output voltage supplied to the furnace. It is observed that the current I1 dies away to zero in a short time based on the low time constant of the circuit containing the secondary 18 of grid transformer 15.

In the arrangement just described the magnitude of the negative voltage impulse derived from potential transformers 62, 12 or 15 is dependent upon the load current conducted by the respective biphase rectifiers. Sometimes it is not desirable that this impulse voltage should vary with load and for obviating this difliculty I provide an arrangement wherein this voltage impulse can be obtained directly from a saturated transformer which delivers a large negative peaked voltage across the auxiliary discharge path 50 and extinguishes the current flowing therein to commutate the current to the next biphase rectifier to operate. In Fig. 8 I have shown a portion of the frequency changer which is a modification of the embodiment of my invention diagrammatically illustrated in Fig. 3. The same reference numerals have been retained for corresponding parts. Biphase rectiher I! is illustrated as supplied with energy from the three phase source through resistor 11, just as shown in Fig. 3. However, a potential transformer 18 has its primary winding 80 connected across resistance 11 and its secondary winding ll connected across the grid-to-cathode circuit of auxiliary discharge valve 50 so as to supply a positive voltage overcoming the negative bias of battery 65 during the entire period that the biphase rectifier I5 is conducting. The time constant of this circuit should be as large as possible so that a substantially rectangular voltage may be applied to the grid of electric discharge valve 50. Phase shifter 61 as in Fig. 3 controls the ignition point of auxiliary electric discharge valve 50 by impressing a potential across resistor H which is connected across the grid circuit of auxiliary discharge valve 50. A saturated transformer 83 supplies the voltage impulse necessary for extinguishing discharge valve 50 at the end of its conducting period. This impulse is impressed across resistor ll.

The operation of the arrangement disclosed in Fig. 8 is very similar to the operation of the arrangement shown in Fig. 3 and will be readily understood by those skilled in the art. The oscillogram illustrated in Fig. 9 represents the same operating conditions of Fig. 8 as are represented by Fig. 6 relative to Fig. 3. In this case I1 has a high positive value for substantially the entire conducting period of .the biphase rectifier i5 so as to retain the grid 55 positive and thus maintain electric discharge valve 50 conducting. The current I2 through grid resistor 31 is substantially the same. However, the voltage F across electric discharge valve 50 contains a positive peak. The grid potential G on control electrode 55 of auxiliary discharge valve 50 is changed only slightly from that illustrated in Fig. 6. The voltage E across discharge valve 2| and the high frequency output voltage D are the same as illustrated in Fig 6 While I have shown and described particular embodiments of my invention as applied to electric valve frequency changing apparatus, it is to be understood that these are merely illustrative of a number of electric valve converting or translating apparatus to which my invention may be applied. -It will, of course, be obvious to those skilled in the art, that changes and modifications may be made without departing from my invention and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

' What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an electric valve frequency changing system comprising an alternating current supply circuit, an alternating current load circuit operating at a frequency higher than that of said supply circuit, transformer means interconnecting said load and supply circuits and comprising a secondary winding connected to said load circuit and a plurality of primary windings each having a plurality of end terminals and a midtap said midtaps being connected to said supply circuit, a plurality of electric valves associated with said end terminals of said primary windings, a direct current circuit interconnecting said electric valves and said supply circuit, and control means for said electric valves including an auxiliary valve for varying the power supplied to said load circuit.

2. In an electric valve frequency changing system a three-phase alternating current supply circuit, a direct current circuit, a biphase rectifier for each phase of said supply circuit connecting said supply circuit with said direct current circuit and comprising an inductive winding and a pair bf discharge valves provided with an anode, a cathode and a control electrode, an alternating current load circuit having a frequency higher than that of said supply circuit and including a winding inductively associated with each of said inductive windings of said biphase rectifiers, and means including an auxiliary valve connected to the control electrodes of said discharge valves for varying the power supplied to said "load circuit.

3. In an electric valve frequency changing system, a polyphase alternating current supply, means for converting said alternating current supply into single phase alternating current of a frequency higher than supply circuit comprising a plurality of biphase rectifiers each including a plurality of main discharge valves and a control system therefor, said control system including an 6 amaeoo auxiliary valve, means for rendering said auxiliary valve conductive at varied instants during a cycle of the alternating current supply for varying the power output of said frequency changing system, and means including a potential transformer for extinguishing the electric discharge in said auxiliary valve to aid commutation from one biphase rectifier to the-other.

4. In an electric valve frequency changing sys- 10 tem, a polyphase alternating current supply voltage, means for converting said supply voltage into alternating current voltage of a higher frequency than that of said supply voltage, including a main electric discharge valve provided with a control circuit, andmeans for controlling the power output of said electric valve frequency changer comprising an auxiliary valve connected in the control circuit of said main valve for controlling the time when said main valve becomes conductive, means for varying the instants during a cycle of the alternating current supply voltage that said main valve becomes conductive, and voltage impulse producing means for extinguishing the current discharge in said auxiliary valve whereby control of the electric discharge in said main valve may again be obtained.

5. In an electric valve frequency changing system, an alternating current supply voltage, means for converting said supply voltage into an alternating current voltage of a higher frequency than that of said supply voltage including-a plurality of'biphase rectifiers each provided with a pair of main electric discharge valves, a control circuit for said electric discharge valves including an auxiliary valve, means for controlling the power output of said frequency changing system by controlling the operation of said auxiliary valve, and-means dependent'upon the initiation of current in a second biphase rectifier 40 for extinguishing the discharge current flowing through said auxiliary valve, and switching means in series with said auxiliary valve for causing said frequency changing system to become I inoperative.

6. In an electric valve frequency changer, a polyphase alternating current supply circuit, an alternating current load circuit, means including a plurality of inductive windings each having a plurality of electric discharge valves associated therewith for transmitting current from said supply circuit to said load circuit at a higher frequency than the frequency of said supply circuit, each of said inductive windings being energized from a different phase of said supply circuit and having end terminals connected to corresponding elements of the electric discharge valves associated therewith, and control means for controlling the portion of the low frequency supply voltage wave during which the valves as- 0 sociated with each of said inductive windings are conductive to vary the voltage of said high frequency load circuit.

7. Control means for an electric valve frequency changer of the type including a plurality 5 of main electric discharge valves provided with control circuits for controlling the power output of said frequency changer including a source of control potential tending to render said main valves nonconductive, an auxiliary valve shunting said source of control potential, means for periodically rendering said valve conductive to remove the eifect of said control potential on said main discharge valves, and switching means in series with said auxiliary valve to prevent said auxiliary valve from becoming conductive.

8. In combination, a low frequency alternating I current supply circuit, an electric valve frequency changer comprising a plurality of biphase rectiflers operating one after the other in timed sequence for converting the low frequency alterg nating current from said supply circuit into a higher frequency alternating. current, and means for varying the power output of said frequency changer comprising a control circuit for maintaining said biphase rectifiers inoperative during 10 different portions of a cycle of said low frequency alternating current supply.

9. In combination, a low frequency alternating current supply circuit, an electric valve frequency changer comprising a plurality of biphase rectill fiers operating one after another in timed sequence for converting the low frequency alternating current from said supply circuit into a higher frequency alternating current, a control circuit for controlling the conductivities of the valves of said frequency changer to vary the power output thereof including a source of variable frequency control potential for determining the frequency of said alternating current load circuit and means for determining the portion of the alternating current voltage wave of said low frequency supply during which the valves of each of said biphase rectifiers are transmitting current at the frequency of said load circuit.

10. In an electric valve frequency changing system, an alternating current supply voltage, means for converting said supply voltage into an alternating current voltage of a higher frequency than that of said supply voltage including a plurality of biphase rectiilers each provided with a pair of main electric discharge valves and an auxiliary valve for controlling the power output of said frequency changing system, means for varying the instant during a cycle of the supply voltage that said auxiliary valve is rendered conductive, and means including a potential transformer for maintaining said auxiliary valve con ductive during the entire time that its associated biphase rectifier is operating. 1

11. In an electric valve frequency changing 5 system, an alternating current supply voltage,

means for converting said supply voltage into an alternating current voltage of a higher frequency than that of said supply voltage including a plurality of biphase rectiflers each provided with 50 a pair of main electric discharge valves and a control circuit for said valves including an auxiliary valve for controlling the power output of said frequency changing system, controllable means for rendering said auxiliary valve con- 55 ductive at different instants during a cycle of the supply voltage for initiating the operation of the associated biphase rectifier, means for maintaining said auxiliary valve conductive during the entire period that its associated biphase rectifier 5 -is to operate, and means for extinguishing the discharge in said auxiliary valve at the instant that its associated biphase rectifier is to become inoperative. j

12. In an electric valve frequency changing 65 system, a polyphase alternating current supply voltage, means for converting said alternating current supply voltage into a single phase alternating current voltage of a higher frequency comprising a plurality of biphase rectifierseach 7 including a plurality of main discharge valves and a control system therefor, said control system including a pair of reversely connected auxiliary valves, control means for rendering one of said auxiliary valves conductive at various instants during a cycle of the alternating current supply voltage for varying the power output of said frequency changing system, means for impressing an impulse voltage across said con trolled auxiliary electric discharge valve to extinguish the electric discharge therethrough at the end of its conducting period, and means including said second auxiliary valve for preventing said voltage impulse from injuring said electric valve frequency changing system.

13. In an electric valve frequency changing system, an alternating current supply voltage, means for converting said voltage into an alternating current voltage of a higher frequency than that of said supply voltage including a pluality of inductive windings each having a plurality of main electric discharge valves associated therewith, a control circuit for controlling the conductivities of the electric discharge valves associated with each of said inductive windings including an auxiliary valve, and means for commutating current from one of said inductive windings to a second one of said inductive windings including means responsive to the initiation of current in 'the valves associated with said second inductive winding for extinguishing the current flowing through the auxiliary valve associated with said one inductive winding.

14. In an electric valve frequency changing system, an alternating current supply voltage, means for converting said supply voltage into an alternating current voltage of a higher frequency including a plurality of inductive windings each having a pair of main electric discharge valves including a control electrode associated therewith, said load circuit including an inductive winding inductively coupled to said first named' inductive windings, a control circuit associated with the control electrodes of said valves includtrol potential tending to render said electric discharge valves alternately conductive and nonconductive at said high frequency, and means for deriving a potential having the frequency of said supply voltage and variable in phase with respect thereto for 'determining the portion of the voltagewave of said supply during which said high frequency control potential is effective to control the conductivities of the valves associated with each of said first named inductive windings to vary the output of said frequency changing system.

15. In an electric valve frequency changing system, an alternating current supply circuit, means including a plurality of inductive windings having a plurality of electric discharge valves including a control electrode associated therewith for transmitting current'from said supply to said load circuit at a higher frequency than the frequency of said supply, each of said inductive windings having and terminals connected to corresponding elements of the electric discharge valve associated therewith, a control circuit associated with the control electrodes of said valves including a source of high frequency alternating current tending to render said electric discharge valves alternately conductive and nonconductive at said high frequency, means for controlling the portion of the voltage wave of said alternating current supply during which the valves associated .with each of said inductive windings is conducting. including an auxiliary discharge valve; and means for rendering said auxiliary valve conductive'at variable times in the alternating current supply voltage wave to determine the portion of the supply voltage wave during which the main valves associated with each of said inductive windings are rendered conductive and nonconductive by said high frequency control potential.

' HELIMUTH 0. 5011mm.

CERTIFICATE O CORRECTION.- Patent No. 2,219,200. October 22, 191m. .HELLMUTH 0. SCHMIDT.

. 'It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2 first column, line 148, before the word "midtap" insert -,-a--; page 3, second col-' line h2',for "is" read --its--; page 14,, first column, line 59, for "f""-' before the read -If-; page 7, first column, line 15-16, claim 15, for

"plnality" read --p1ura1ity--; and that the soid Letters Patent should 'be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 10th day of December, A. D. 1914.0.

Henry Van Arsdale (Seal) Acting Commissioner of Patenti.

CERTIFICATE 0 CORRECTION; Patent No. 2,219,200. October 22, 191w. -.HELLHUTH o. SCHMIDT.

-It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2 first column, line h8, before the word "midtap" insert -a-; page 5, second col? line .2',for "is" read -its--; page )4, first column, line 59, for "f" v before "the" read --If-; page 7, first colximn, line 15-16 claim 15, for

"pluality" read --plurality--; and that the said Letters Patent shouldjbe read with this correction therein that the same may conform to the record 7 of the case in the Patent Office.

Signed and sealed this 10th day of December, A. D. 191m.

Henry Van -Arsdale (Seal) Acting Commissioner of Paten tk. 

